The present invention relates to a liquid crystal display device, and more particularly, it relates to a liquid crystal display device having a wide viewing angle characteristic and high display quality.
Recently, a thin and light liquid crystal display device is used as a display device for a display of a personal computer or a display unit of portable information terminal equipment. Conventional twist nematic (TN) or super twist nematic (STN) liquid crystal display devices have, however, a disadvantage of a narrow viewing angle, and a variety of techniques have been developed for overcoming this disadvantage.
A typical technique to improve the viewing angle characteristic of a TN or STN liquid crystal display device is a method of additionally providing an optical compensator. Another technique is a lateral field method of applying, through a liquid crystal layer, an electric field in a lateral direction to the substrate surface. Liquid crystal display devices of the lateral field method are recently mass-produced and regarded as promising devices. A still another technique is DAP (deformation of vertical aligned phase) in which a nematic liquid crystal material with negative dielectric anisotropy is used as a liquid crystal material and a vertical alignment film is used as an alignment film. The DAP is a kind of an electrically control birefringence (ECB) method, in which the transmittance is controlled by utilizing the birefringent property of the liquid crystal molecules.
Although the lateral field method is one of the effective methods for attaining a wide viewing angle, the production margin is very small in the production process as compared with that of a general TN liquid crystal display device, and hence, there is a difficulty in stable production of this type of liquid crystal display devices. This is because gap irregularity between substrates and shift of the transmission axis of a polarizing plate (polarization axis) from the orientation axis of a liquid crystal molecule largely affect the luminance and the contrast ratio of display. In order to stably produce the liquid crystal display devices of the lateral field method by highly precisely controlling these factors, the technique should be further highly developed.
Furthermore, in order to produce an even display free from display unevenness by a liquid crystal display device of the DAP method, it is necessary to control orientation. For controlling the orientation, an alignment treatment is carried out by rubbing the surface of an alignment film. When the surface of a vertical alignment film is subjected to a rubbing treatment, however, rubbing streaks are easily caused in a displayed image. Therefore, this treatment is not suitable to mass-production.
On the other hand, for controlling the orientation without the rubbing treatment, a method for controlling the orientation directions of liquid crystal molecules by an inclined electric field generated by forming a slit (opening) in an electrode has been proposed (as described in, for example, Japanese Laid-Open Patent Publication Nos. 6-301036 and 2000-47217). However, the present inventors have found the following as a result of examination: The orientation state of a portion of a liquid crystal layer corresponding to the opening of the electrode is not specified in the methods disclosed in these publications, and the continuity of the orientation of the liquid crystal molecules is not sufficient. Therefore, it is difficult to obtain a stable orientation state over an entire picture element, and hence, a displayed image becomes disadvantageously uneven.
The present invention was devised to overcome the aforementioned disadvantages, and an object of the invention is providing a liquid crystal display device having a wide viewing angle characteristic and high display quality.
The liquid crystal display device of this invention includes a first substrate; a second substrate; a liquid crystal layer disposed between the first substrate and the second substrate; and a plurality of picture element regions each defined by a first electrode provided on a surface of the first substrate facing the liquid crystal layer and a second electrode provided on a surface of the second substrate facing the liquid crystal layer, and the first substrate has, on the surface thereof facing the liquid crystal layer, at least one first protrusion with an inclined side face correspondingly to each of the plurality of picture element regions, and a portion of the liquid crystal layer included in each of the plurality of picture element regions is in a substantially vertical orientation state under application of no voltage, and includes at least a part of a first liquid crystal domain placed in a radially-inclined orientation state about the at least one first protrusion under voltage application, for producing a display by changing an orientation state of the liquid crystal layer in accordance with an applied voltage. Thus, the aforementioned object can be achieved.
The at least one first protrusion may be formed within each of the plurality of picture element regions.
The at least one first protrusion may be plural in number, and the portion of the liquid crystal layer included in each of the plurality of picture element regions may include a plurality of first liquid crystal domains all placed in the radially-inclined orientation state under voltage application.
The first electrode may include at least one first opening, and the at least one first protrusion may be formed within the at least one first opening.
Preferably, the second substrate has, on the surface thereof facing the liquid crystal layer, at least one second protrusion with an inclined side face correspondingly to each of the plurality of picture element regions, and the portion of the liquid crystal layer included in each of the plurality of picture element regions includes, under voltage application, at least a part of a second liquid crystal domain placed in a radially-inclined orientation state about the at least one second protrusion, so that inclination directions of liquid crystal molecules in the first liquid crystal domain can be continuous with inclination directions of liquid crystal molecules in the second liquid crystal domain.
The second electrode may have at least one second opening, the portion of the liquid crystal layer included in each of the plurality of picture element regions may include, under voltage application, a second liquid crystal domain placed in a radially-inclined orientation state about the at least one second opening, so that inclination directions of liquid crystal molecules in the first liquid crystal domain can be continuous with inclination directions of liquid crystal molecules in the second liquid crystal domain.
The second electrode may have at least one second opening, and the at least one second protrusion may be formed within the at least one second opening.
The at least one second protrusion may include a plurality of second protrusions formed out of each of the plurality of picture element regions.
A cross-section, taken along a surface of the first substrate, of the at least one first protrusion is preferably in a shape having rotational symmetry.
Alternatively, the cross-section, taken along the surface of the first substrate, of the at least one first protrusion may be in a substantially circular shape.
Alternatively, the cross-section, taken along the surface of the first substrate, of the at least one first protrusion may be in a substantially cross shape consisting of crossing lines extending along a first direction and a second direction crossing each other at substantially right angles.
The liquid crystal display device may further include a pair of polarizing plates respectively provided on outer surfaces of the first substrate and the second substrate, so that the pair of polarizing plates may be disposed in such a manner that a polarization axis of one of the pair of polarizing plates is parallel to the first direction and a polarization axis of the other of the pair of polarizing plates is parallel to the second direction.
A shape of the at least one first opening seen from a normal direction of the first substrate preferably has rotational symmetry.
A cross-section, taken along a surface of the second substrate, of the at least one second protrusion is preferably in a shape having rotational symmetry.
A shape of the at least one second opening seen from a normal direction of the second substrate preferably has rotational symmetry.
Preferably, the at least one first protrusion is plural in number, and at least some of the plural first protrusions are arranged so as to have rotational symmetry.
Preferably, the at least one first opening is plural in number, and at least some of the plural first openings are arranged so as to have rotational symmetry.
Preferably, the at least one second protrusion is plural in number, and at least some of the plural second protrusions are arranged so as to have rotational symmetry.
Preferably, the at least one second opening is plural in number, and at least some of the plural second openings are arranged so as to have rotational symmetry.
The inclined side face of the first protrusion and/or the second protrusion is inclined at an angle of preferably 5 degrees through 85 degrees and more preferably 50 degrees or less against the surface of the first substrate and/or the surface of the second substrate.
Each of the plurality of picture element regions may have a plurality of portions having different thicknesses of the liquid crystal layer, at least one of the first substrate and the second substrate may have level differences between the plurality of portions, and the level differences may be covered with the first electrode or the second electrode. In this case, at least some of the at least one first protrusion is preferably surrounded with the level differences.
This structure is effectively employed particularly in a liquid crystal display device in which the first electrode includes a transparent electrode and a reflecting electrode, each of the plurality of picture element regions includes a transmission region for producing a display in a transmission mode and a reflection region for producing a display in a reflection mode, and the liquid crystal layer has a larger thickness in the transmission region than in the reflection region.
The first substrate may further include an active element provided correspondingly to each of the plurality of picture element regions, the first electrode may correspond to picture element electrodes respectively provided in the plurality of picture element regions to be switched by the active element, and the second electrode may correspond to at least one counter electrode opposing the picture element electrodes. The counter electrode is typically formed as a single electrode extending over an entire display region.
The second substrate may further include an active element provided correspondingly to each of the plurality of picture element regions, the second electrode may correspond to picture element electrodes respectively provided in the plurality of picture element regions to be switched by the active element, and the first electrode may correspond to at least one counter electrode opposing the picture element electrodes.
The functions of the invention are as follows:
The liquid crystal display device of the invention is a liquid crystal display device of a vertical orientation mode in which the liquid crystal layer is substantially in a vertical orientation state under application of no voltage. The vertical alignment type liquid crystal layer is obtained typically by orienting a nematic liquid crystal material having negative dielectric anisotropy with a vertical alignment film. A plurality of protrusions each having an inclined side face are provided on one of the pair of substrates disposed so as to sandwich the liquid crystal layer (for example, a TFT substrate). Since liquid crystal molecules are oriented vertically to the inclined side face (typically covered with a vertical alignment film) of the protrusion, liquid crystal molecules present around the protrusion are inclined radially about the protrusion. Most liquid crystal molecules other than those present in the vicinity of the inclined side face of the protrusion are in the vertical orientation state.
When a voltage is applied through the liquid crystal layer, liquid crystal molecules are inclined in directions matching with the orientation directions of the liquid crystal molecules inclined due to the influence (orientation-regulating force or the so-called anchoring effect) of the inclined side face of the protrusion. The extent of the inclination of the liquid crystal molecule (i.e., the inclination angle) depends upon the strength of the electric field, and as the electric field is stronger, the liquid crystal molecule is more largely inclined to be oriented in a direction closer to the horizontal direction. The inclination direction of the liquid crystal molecule accords with the inclination direction of the liquid crystal molecule inclined radially about the protrusion by the anchoring effect of the inclined side face of the protrusion. Therefore, under voltage application, a liquid crystal domain in a radially-inclined orientation state is formed in the liquid crystal layer. In the liquid crystal domain placed in the radially-inclined orientation state, the liquid crystal molecules are oriented along all the azimuth directions. As a result, the viewing angle characteristic of the liquid crystal display device can be improved in all the azimuth directions.
The plurality of protrusions are provided correspondingly to picture element regions, so that domains with the radially-inclined orientation can be formed in the respective picture element regions in the liquid crystal layer. For example, at least one protrusion is provided in each picture element region so as to form a domain with the radially-inclined orientation about the protrusion in the picture element region in the liquid crystal layer. Alternatively, a plurality of protrusions are provided in the periphery of the picture element region (for example, a portion corresponding to a source line, a gate line or the like), so that the picture element region in the liquid crystal layer can include a set of parts of a plurality of domains with the radially-inclined orientation respectively formed about the protrusions. Needless to say, these two structures may be combined.
In the liquid crystal display device of this invention, the domain with the radially-inclined orientation is formed by utilizing the orientation-regulating force of the inclined side face of the protrusion. Since the orientation-regulating force caused by the inclined side face works also under application of no voltage, even if the orientation of the liquid crystal layer is disturbed, for example, due to impact against the liquid crystal display device, the radially-inclined orientation about the protrusion can be restored when external force applied to the liquid crystal material is removed. Accordingly, the liquid crystal display device of this invention is advantageous to a conventional liquid crystal display device in which the radially-inclined orientation is formed by utilizing an inclined electric field generated by an electrode having an opening (slit).
The display characteristic of a liquid crystal display device exhibits azimuth angle dependency derived from the orientation state (optical anisotropy) of liquid crystal molecules. In order to reduce the azimuth angle dependency of the display characteristic, the liquid crystal molecules are preferably oriented along the respective azimuth directions in equivalent probabilities. Furthermore, the liquid crystal molecules within each picture element region are preferably oriented along the respective azimuth directions in equivalent probabilities. Accordingly, the protrusion preferably has such a shape that the liquid crystal domains can be formed so as to orient the liquid crystal molecules in each picture element region along the respective azimuth directions in equivalent probabilities.
When the cross-section, taken along the substrate surface, of the protrusion has rotational symmetry, the viewing angle characteristic can be made uniform along all the azimuth directions. The cross-sectional shape preferably has highly rotational symmetry with a two-fold rotation axis or more preferably with a rotation axis of four or more folds (as in, for example, a square and a circle).
Furthermore, as the area of the inclined side face of the protrusion is larger, the orientation-regulating force against the liquid crystal molecules is larger. For example, when the protrusion has a substantially cross-shaped cross-section, the area of the inclined side face can be comparatively increased, so as to comparatively increase the orientation-regulating force against the liquid crystal molecules. Therefore, the radially-inclined orientation can be further stabilized and the response speed can be increased. Moreover, when the protrusion has a substantially cross-shaped cross-section, the transmittance and the contrast ratio can be also improved by allowing the polarization axis directions of a pair of polarizing plates disposed in a crossed Nicols state to accord with the directions of the crossing lines of the cross (i.e., directions crossing each other at substantially right angles).
When a plurality of protrusions are provided, the liquid crystal domains with the radially-inclined orientation can be uniformly formed by disposing the plurality of protrusions in rotationally symmetrical arrangement (for example, in square lattice arrangement).
The orientation of the liquid crystal molecules can be further stabilized by utilizing, in addition to the anchoring effect of the inclined side face of the protrusion, orientation-regulating force caused by an inclined electric field generated by an electrode having an opening. When the protrusion is formed within the opening of the electrode, the direction of the orientation-regulating force caused by the inclined electric field can accord with the direction of the orientation-regulating force caused by the inclined side face, and hence, the liquid crystal molecules can be stably placed in the radially-inclined orientation state. The shape of the opening seen from the normal direction also preferably has rotational symmetry and is preferably the same as (similar to) the cross-sectional shape of the protrusion. Needless to say, the opening may be disposed in a position different from the protrusion. However, in the case where a plurality of openings are provided, they are preferably disposed so as to have rotational symmetry. Also, the protrusion and the opening are preferably disposed in one arrangement having complementary rotational symmetry. For example, assuming that the opening is replaced with the protrusion, they are preferably disposed so that a plurality of protrusions including the replaced protrusion can have rotational symmetry.
In the case where a plurality of protrusions and/or openings are provided in one picture element region, it is not always necessary to arrange them so as to have rotational symmetry over the entire picture element region. For example, when a square lattice (symmetrical with a four-fold rotation axis) is used as a minimum unit so as to form a picture element region from the combination of the square lattices, the liquid crystal molecules can be oriented along all the azimuth directions in substantially equivalent probabilities all over the picture element region. In other words, a portion of the liquid crystal layer included in each picture element region may be formed as a set of liquid crystal domains arranged so as to have rotational symmetry (or axial symmetry) (for example, a plurality of liquid crystal domains in square lattice arrangement).
In the liquid crystal display device of this invention, the orientation of the liquid crystal molecules can be further stabilized by providing protrusions and/or openings also on a second substrate (for example, a counter substrate or a color filter substrate) opposing a first substrate on which the protrusions and/or openings are provided. Owing to orientation-regulating force caused by the protrusions/or openings provided on the surface of the second substrate facing the liquid crystal layer, liquid crystal domains with the radially-inclined orientation are formed under voltage application.
The radially-inclined orientation about each protrusion and/or opening of the second substrate is preferably formed so as to be continuous with the radially-inclined orientation about each protrusion and/or opening of the first substrate. For this purpose, when seen from a vertical direction to the substrate, the protrusions and/or openings provided on the first substrate are preferably arranged so as not to overlap the protrusions and/or openings provided on the second substrate. The respective protrusions and/or openings are preferably arranged so as to have rotational symmetry as described above. Accordingly, when they are disposed in square lattice arrangement, the protrusions and/or openings provided on the respective substrates are disposed so that the lattice points of the square lattices formed by the protrusions and/or openings of the second substrate are respectively positioned at the centers of the square lattices formed by the protrusions and/or openings of the first substrate. Needless to say, the first substrate and the second substrate may be replaced with each other.
Since light leakage may be caused in a portion corresponding to the protrusion, the protrusion is preferably provided in the periphery of the picture element region (for example, in a portion corresponding to a scanning line or a signal line) or in a region corresponding to an element not transmitting light such as an storage capacitance line included in the picture element region. When the protrusions are disposed in such portions, degradation of the display quality can be suppressed.
In the liquid crystal display device of this invention, the protrusion is formed at least on one of the substrates (for example, a TFT substrate or a color substrate), and hence, a stable liquid crystal domain placed in the radially-inclined orientation state under voltage application can be formed in the vertical alignment type liquid crystal layer.
In particular, in a liquid crystal display device of the so-called multi-gap system having different thicknesses of the liquid crystal layer within one picture element region, such as a transmission/reflection combination type liquid crystal display device having a transmission region and a reflection region in each picture element region (as disclosed in, for example, Japanese Laid-Open Patent Publication No. 11-101992), the orientation of the liquid crystal molecules is easily disturbed due to the influence of a level difference. Therefore, in such a liquid crystal display device, a liquid crystal domain with sufficiently stable radially-inclined orientation is difficult to form merely by using the orientation-regulating force caused by an inclined electric field. According to the invention, however, discontinuity in the orientation of the liquid crystal molecules owing to the level difference is suppressed by an electric field generated by an electrode covering the level difference, and the protrusion with an appropriate inclined side face is provided so as to form the center of the radially-inclined orientation by using the orientation-regulating force caused by the inclined side face. As a result, stable radially-inclined orientation can be realized. In particular, when the protrusion is surrounded with the level differences covered with the electrode, the discontinuity in the orientation of the liquid crystal molecules owing to the level differences can be effectively suppressed.
In this manner, the viewing angle characteristic of a liquid crystal display device can be improved by the present invention. Therefore, when the invention is applied to an active matrix liquid crystal display device in particular, a display with very high quality can be produced.